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Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Fixed maturities
Amortized Cost	$ 1,866.4	$ 2,032.9
Gross Unrealized Gains	52.5	40.0
Gross Unrealized Losses	1.9	3.1
Allowance for Credit Losses	1.2	0.9	$ 0.2	$ 0.2
Fair Value	1,915.8	2,068.9
U.S. Governments
Fixed maturities
Amortized Cost	210.5	222.3
Gross Unrealized Gains	3.0	1.5
Gross Unrealized Losses	0.3	0.6
Allowance for Credit Losses	0.0	0.0
Fair Value	213.2	223.2
Foreign Governments
Fixed maturities
Amortized Cost	4.1	7.1
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	4.2	7.1
Obligations of states and political subdivisions
Fixed maturities
Amortized Cost	66.2	80.1
Gross Unrealized Gains	1.9	1.6
Gross Unrealized Losses	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	68.1	81.7
Corporate bonds
Fixed maturities
Amortized Cost	896.4	1,033.5
Gross Unrealized Gains	23.3	19.4
Gross Unrealized Losses	1.2	1.3
Allowance for Credit Losses	1.1	0.9
Fair Value	917.4	1,050.7
Commercial mortgage-backed securities
Fixed maturities
Amortized Cost	305.1	276.6
Gross Unrealized Gains	12.6	10.0
Gross Unrealized Losses	0.3	0.3
Allowance for Credit Losses	0.0	0.0
Fair Value	317.4	286.3
Residential mortgage-backed securities
Fixed maturities
Amortized Cost	196.7	201.0
Gross Unrealized Gains	5.7	2.8
Gross Unrealized Losses	0.0	0.6
Allowance for Credit Losses	0.0	0.0
Fair Value	202.4	203.2
Asset-backed securities
Fixed maturities
Amortized Cost	101.1	104.9
Gross Unrealized Gains	3.0	2.6
Gross Unrealized Losses	0.1	0.1
Allowance for Credit Losses	0.1	0.0
Fair Value	103.9	107.4
Collateralized loan obligations
Fixed maturities
Amortized Cost	86.3	107.4
Gross Unrealized Gains	2.9	2.1
Gross Unrealized Losses	0.0	0.2
Allowance for Credit Losses	0.0	0.0
Fair Value	$ 89.2	$ 109.3